Hello all:

I'm new to the forum. I have a 94 C220/70K that I would like to test if the MAS sensor ir defective or not. I have heard (From MB Tech) that one simple diagnostic method to determine this, is to unscrew the MAS sensor from the housing unit and lightly tap on it with a screwdriver. Once this is done, if you find that your engine surges- that replacement is needed.

Secondly, Although I'm not real mechanically inclined, I have been unable to figure out how to loosen the two screws that hold the MAS in place. They seem to have almost like a wheel spoke pattern design with a small hole in the center of the screw. Is there a special tool needed If so, where do I purchse? I have tried all different types with no success.

Thirdly, which O2 sensor (front or back of cat) is responsible for malfunctioning or surging and or poor fuel consumption. THanks to all in advance.

The screws are anti-tamper torx and they are available althouth the latest AMM have a new 5 sided unit that I've never seen the tool for. There is no need to unscrew the capsule portion for replacement though the whole assembly is sold as a unit.

Your diagnostic technique would be real appropriate if you take a 5lb hammer and smash it. If it changes the way it runs replace it That I would agree with. If you have an inclination for testing with meaning see what the Key on engine off output is. A good unit is 1v. Further testing would use the adaptive values from actual values.

The forward O2 sensor is the primary mixture feedback the second one is mostly for evaluating the efficiency of the cat although there is a secondary mixture control available at times, not well defined.

__________________
Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician

Hi Steve:

Thanks for the information, I knew that I was doing something wrong.

So please tell me, other than having the diagnostic codes read by MB, how do I determine if both the O2 sensor and the MAS are malfunctioning? I'm experiencing surging at various times, difficulty starting, and poor fuel performance. Oh yes, and a high frustration level. :-)